Process for producing para-xylene

ABSTRACT

A process for producing a PX-rich product comprises (a) separating a feedstock containing C 8  hydrocarbons to produce a C 8  hydrocarbons rich stream; (b) separating at least a first portion of the C 8  hydrocarbons rich stream to produce a first PX-rich stream and a first PX-depleted stream; (c) isomerizing at least a portion of the first PX-depleted stream to produce a first isomerized stream having a higher PX concentration than the first PX-depleted stream; (d) separating a second portion of the C 8  hydrocarbons rich stream and/or at least a portion of the first isomerized stream to produce a second PX-rich stream and a second PX-depleted stream; (e) isomerizing at least a portion of the second PX-depleted stream to produce a second isomerized stream having a higher PX concentration than the second PX-depleted stream; (f) recovering at least a portion of at least one of the first and second PX-rich streams as PX-rich product; and (g) supplying at least a portion of at least one of the first isomerized stream, the second isomerized stream, the first PX-rich stream, and the second PX-rich stream to the separating (a).

PRIORITY CLAIM

This application claims the benefit of U.S. Provisional application No.60/794,864, filed Apr. 25, 2006.

FIELD OF THE INVENTION

This invention relates to a process for producing para-xylene.

BACKGROUND OF THE INVENTION

Ethylbenzene (EB), para-xylene (PX), ortho-xylene (OX) and meta-xylene(MX) are often present together in C₈ aromatic product streams fromchemical plants and oil refineries. Of these C₈ compounds, although EBis an important raw material for the production of styrene, for avariety of reasons most EB feedstocks used in styrene production areproduced by alkylation of benzene with ethylene, rather than by recoveryfrom a C₈ aromatics stream. Of the three xylene isomers, PX has thelargest commercial market and is used primarily for manufacturingterephthalic acid and terephthalate esters for use in the production ofvarious polymers such as poly(ethylene terephthalate), poly(propyleneterephthalate), and poly(butene terephthalate). While OX and MX areuseful as solvents and raw materials for making products such asphthalic anhydride and isophthalic acid, market demand for OX and MX andtheir downstream derivatives is much smaller than that for PX.

Given the higher demand for PX as compared with its other isomers, thereis significant commercial interest in maximizing PX production from anygiven source of C₈ aromatic materials. However, there are two majortechnical challenges in achieving this goal of maximizing PX yield.Firstly, the four C₈ aromatic compounds, particularly the three xyleneisomers, are usually present in concentrations dictated by thethermodynamics of production of the C₈ aromatic stream in a particularplant or refinery. As a result, the PX production is limited, at most,to the amount originally present in the C₈ aromatic stream unlessadditional processing steps are used to increase the amount of PX and/orto improve the PX recovery efficiency. Secondly, the C₈ aromatics aredifficult to separate due to their similar chemical structures andphysical properties and identical molecular weights.

A variety of methods are known to increase the concentration of PX in aC₈ aromatics stream. These methods normally involve recycling the streambetween a separation step, in which at least part of the PX is recoveredto produce a PX-depleted stream, and a xylene isomerization step, inwhich the PX content of the PX-depleted stream is returned back towardsequilibrium concentration, typically by contact with a molecular sievecatalyst. However, the commercial utility of these methods depends onthe efficiency, cost effectiveness and rapidity of the separation stepwhich, as discussed above, is complicated by the chemical and physicalsimilarity of the different C₈ isomers.

Fractional distillation is a commonly used method for separatingdifferent components in chemical mixture. However, it is difficult touse conventional fractional distillation technologies to separate EB andthe different xylene isomers because the boiling points of the four C₈aromatics fall within a very narrow 8° C. range, namely from about 136°C. to about 144° C. (see Table 1 below). In particular, the boilingpoints of PX and EB are about 2° C. apart, whereas the boiling points ofPX and MX are only about 1° C. apart. As a result, large equipment,significant energy consumption, and/or substantial recycles would berequired for fractional distillation to provide effective C₈ aromaticseparation.

TABLE I C₈ compound Boiling Point (° C.) Freezing Point (° C.) EB 136−95 PX 138 13 MX 139 −48 OX 144 −25

Fractional crystallization is an alternative method of separatingcomponents of a mixture and takes advantage of the differences betweenthe freezing points and solubilities of the components at differenttemperatures. Due to its relatively higher freezing point, PX can beseparated as a solid from a C₈ aromatic stream by fractionalcrystallization while the other components are recovered in aPX-depleted filtrate. High PX purity, a key property needed forsatisfactory conversion of PX to terephthalic acid and terephthalateesters, can be obtained by this type of fractional crystallization. U.S.Pat. No. 4,120,911 provides a description of this method. Commerciallyavailable fractional crystallization processes and apparatus include thecrystallization isofining process, the continuous countercurrentcrystallization process, direct CO₂ crystallizer, and scraped drumcrystallizers. Due to high utility usage and the formation of a eutecticbetween PX and MX, it is usually more advantageous to use a feed with ashigh an initial PX concentration as possible when using fractionalcrystallization to recover PX.

An alternative xylene separation method uses molecular sieves, such aszeolites, to selectively adsorb para-xylene from the C₈ aromaticfeedstream to form a PX-depleted effluent. The adsorbed PX can then bedesorbed by various ways such as heating, lowering the PX partialpressure or stripping. (See generally U.S. Pat. Nos. 3,706,812,3,732,325 and 4,886,929) Two commercially available processes used inmany chemical plants or refineries are PAREX™ and ELUXYL™ processes.Both processes use molecular sieves to adsorb PX. In suchmolecular-sieve based adsorption processes, a higher amount of PX,typically over 90%, compared with that from a fractional crystallizationprocess, typically below 65%, may be recovered from the PX present in aparticular feed.

For many of these PX separation processes, the higher the original PXconcentration in the feed stream, the easier, more efficient and moreeconomical it becomes to perform the PX separation. Therefore, there arestrong economic and technical incentives to increase the PXconcentration in a hydrocarbon feed stream comprising the C₈ aromaticcompounds prior to sending the feed stream to a PX recovery unit.

There is, therefore a need for an improved process for increasing the PXconcentration in C₈ aromatic streams prior to sending the streams to thePX recovery units. This higher PX concentration would also allow betterutilization and/or de-bottlenecking of existing PX separation equipment,such as a PAREX™ unit, an ELUXYL™ unit or a fractional crystallizer.

SUMMARY OF THE INVENTION

In one aspect, the present application describes a process for producinga PX-rich product, the process comprising:

-   -   (a) separating a feedstock containing C₈ hydrocarbons to produce        a C₈ hydrocarbons rich stream;    -   (b) separating at least a first portion of the C₈ hydrocarbons        rich stream to produce a first PX-rich stream and a first        PX-depleted stream;    -   (c) isomerizing at least a portion of the first PX-depleted        stream to produce a first isomerized stream having a higher PX        concentration than the first PX-depleted stream;    -   (d) separating a second portion of the C₈ hydrocarbons rich        stream and/or at least a portion of the first isomerized stream        to produce a second PX-rich stream and a second PX-depleted        stream;    -   (e) isomerizing at least a portion of the second PX-depleted        stream to produce a second isomerized stream having a higher PX        concentration than the second PX-depleted stream;    -   (f) recovering at least a portion of at, least one of the first        and second PX-rich streams as PX-rich product; and    -   (g) supplying at least a portion of at least one of the first        isomerized stream, the second isomerized stream, the first        PX-rich stream, and the second PX-rich stream to the separating        (a).

Conveniently, the feedstock contains at least C₈+ hydrocarbons and theseparating (a) produces the C₈ hydrocarbons rich stream and a C₉+hydrocarbons rich stream.

In another aspect, the present application describes a process forproducing a PX-rich stream, the process comprising:

-   -   (a) separating a feedstock containing C₈ hydrocarbons to produce        a C₈ hydrocarbons rich stream;    -   (b) separating at least a portion of the C₈ hydrocarbons rich        stream to produce the PX-rich stream and a first stream;    -   (c) isomerizing at least a portion of the first stream to        produce a second stream having a higher PX concentration than        the first stream;    -   (d) separating at least a portion of the second stream to        produce a third stream and a fourth stream, the third stream        having a higher PX concentration than the second stream and the        fourth stream having a lower PX concentration than the second        stream;    -   (e) isomerizing at least a portion of the fourth stream to        produce a fifth stream having a higher PX concentration than the        fourth stream; and    -   (f) providing at least a portion of the third stream and/or at        least a portion of the fifth stream to the separating step (a).

Additionally, the process may comprise recycling a portion of the fifthstream and/or a portion of the third stream to (d). Further, the processmay comprise recycling a portion of the fourth stream to (c).

In one embodiment, the process further comprises fractionating saidsecond stream to produce a first portion rich in C₇− hydrocarbons and asecond portion rich in C₈+ hydrocarbons, said second portion beingsupplied to said separating (d).

Conveniently, the separating (b) comprises at least one of selectiveadsorption, selective crystallization, selective extraction, andselective membrane separation, and the separating (d) comprises at leastone of selective adsorption, selective crystallization, selectiveextraction, and selective membrane separation.

In yet another aspect, the present application describes a process forproducing a PX-rich stream, the process comprising:

(a) separating a feedstock containing C₈ hydrocarbons to produce a C₈hydrocarbons rich stream;

(b) separating at least a portion of the C₈ hydrocarbons rich stream toproduce a first stream and a second stream, the first stream having ahigher PX concentration than the C₈ hydrocarbons rich stream and thesecond stream having a lower PX concentration than the C₈ hydrocarbonsrich stream;

(c) isomerizing at least a portion of the second stream to produce athird stream having a higher PX concentration than the second stream;

(d) separating at least a portion of the first stream and/or at least aportion of the third stream to produce the PX-rich stream and a fourthstream;

(e) isomerizing at least a portion of the fourth stream to produce afifth stream having a higher PX concentration than the fourth stream;and

(f) providing at least a portion of the fifth stream to the separatingstep (a).

Additionally, the process may comprise recycling at least a portion ofthe second stream to step (e). Further, the process may compriserecycling at least a portion of the first stream and/or at least aportion of the third stream to step (b).

In one embodiment, the process further comprises fractionating saidfifth stream to produce a first portion rich in C₇− hydrocarbons and asecond portion rich in C₈+ hydrocarbons, said second portion beingsupplied to said separating (a).

In another embodiment, at least a portion of the third stream isprovided to the separating (a).

In this process, the PX-rich stream typically comprises at least 50 wt.% PX, generally at least 90 wt. % PX.

In a further aspect, the present application describes a process forproducing a PX-rich product, the process comprising:

(a) separating a feedstock containing C₈ hydrocarbons to produce a C₈hydrocarbons rich stream;

(b) separating a first portion of the C₈ hydrocarbons rich stream toproduce a first PX-rich stream and a first stream;

(c) isomerizing at least a portion of the first stream to produce asecond stream having a higher PX concentration than the first stream;

(d) separating a second portion of the C₈ hydrocarbons rich stream toproduce a second PX-rich stream and a third stream;

(e) isomerizing at least a portion of the third stream to produce afourth stream having a higher PX concentration than the third stream;

(f) recovering at least a portion of at least one of the first andsecond PX-rich streams as PX-rich product; and

(g) providing at least a portion of the second stream and at least aportion of the fourth stream to the separating (a).

Conveniently, said isomerizing (e) is effected at least partially in theliquid phase.

In one embodiment, the separating (b) comprises selective adsorption andthe separating (d) comprises fractional crystallization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional xylene separation andisomerization loop.

FIG. 2 is a schematic diagram of a process for producing para-xylene inaccordance with a first embodiment of this disclosure.

FIG. 3 is a schematic diagram of a process for producing para-xylene inaccordance with a second embodiment of this disclosure.

FIG. 4 is a schematic diagram of a further conventional xyleneseparation and isomerization loop.

FIG. 5 is a schematic diagram of a process for producing para-xylene inaccordance with a third embodiment of this disclosure.

DETAILED DESCRIPTION OF THE INVENTION

All patents, patent applications, test procedures, priority documents,articles, publications, manuals, and other documents cited herein arefully incorporated by reference to the extent such disclosure is notinconsistent with the present application and for all jurisdictions inwhich such incorporation is permitted.

When numerical lower limits and numerical upper limits are listedherein, ranges from any lower limit to any upper limit are contemplated.

One having ordinary skill in the art understands that the embodimentsdiscussed in this application do not represent all the possibleapparatus or process variations embodied by the present disclosure. Inaddition, many pieces of equipment and apparatus and certain processingsteps may be needed for industrial, commercial or even experimentalpurposes. Examples of such equipments and apparatus and processing stepsare, but not limited to, distillation columns, fractionation columns,heat exchanges, pumps, valves, pressure gauges, temperature gauges,liquid-vapor separators, feed and product driers and/or treaters, claytreaters, feed and/or product storage facilities, and processes andsteps for process control. While such equipment, apparatus and stepsthat are not needed for understanding the essence of the presentapplication are not shown in the drawings, some of them may be mentionedfrom time to time to illustrate various aspects of the disclosure. It isalso noted that some of the equipment may be placed at different placesin the process depending on the conditions of the processes.

As used herein, the term “C₈+ hydrocarbons” means hydrocarbons havingeight or more carbon atoms per molecule. A C₈+ hydrocarbons feed and/orproduct is a hydrocarbon feed and/or product having more than 10 wt. %,such as more than 20 wt. %, for example more than 40 wt. %, such as morethan 50 wt. %, and in some cases more than 80 wt. %, C₈+ hydrocarbons inthe feed and/or product. The term “C₉+ hydrocarbons” as used hereinmeans hydrocarbons having nine or more carbon atoms per molecule. A C₉+hydrocarbons feed and/or product is a hydrocarbon feed and/or producthaving more than 10 wt. %, such as more than 20 wt. %, for example morethan 40 wt. %, such as more than 50 wt. %, and in some cases more than80 wt. %, C₉+ hydrocarbons in the feed and/or product. The term “C₇−hydrocarbons” as used herein means hydrocarbons having seven or lesscarbon atoms per molecule. A C₇− hydrocarbons feed and/or product is ahydrocarbon feed and/or product having more than 10 wt. %, such as morethan 20 wt. %, for example more than 40 wt. %, such as more than 50 wt.%, and in some cases more than 80 wt. %, C₇− hydrocarbons in the feedand/or product. The term “C₈ hydrocarbons” as used herein meanshydrocarbons having eight carbon atoms per molecule, including PX. A C₈hydrocarbons feed and/or product, with the exception of a PX-rich orPX-depleted stream and/or product, is a hydrocarbon feed and/or producthaving more than 10 wt. %, such as more than 20 wt. %, for example morethan 40 wt. %, such as more than 50 wt. %, and in some cases more than80 wt. %, C₈ hydrocarbons in the feed and/or product. The term “C₈aromatic hydrocarbons” as used herein means aromatic hydrocarbons havingeight carbon atoms per molecule, i.e., xylene(s) and/or EB. A C₈aromatic hydrocarbons feed and/or product, with the exception of aPX-rich or PX-depleted stream and/or product, is a hydrocarbon feedand/or product having more than 10 wt. %, such as more than 20 wt. %,for example more than 40 wt. %, such as more than 50 wt. %, and in somecases more than 80 wt. %, C₈ aromatic hydrocarbons in the feed and/orproduct.

The term “PX-depleted” means that PX concentration in an exiting streamof a particular unit is lowered as compared to the concentration in afeed stream to the same unit. It does not mean that all of PX has to bedepleted or removed from the xylenes-containing feed stream(s) to theunit. The term “PX-rich” means that PX concentration in an exitingstream of a particular unit is increased as compared to theconcentration in a feed stream to the same unit. It does not mean thatthe PX concentration has to be 100%.

Feedstock

The feedstock employed in the present process may be any C₈+ hydrocarbonfeedstock containing C₈ aromatic hydrocarbons, such as a reformatestream, a hydrocracking product stream, a xylene or EB reaction productstream, an aromatic alkylation product stream, an aromaticdisproportionation stream, an aromatic transalkylation stream, and/or aCyclar™ process stream. The feedstock may further comprise recyclestream(s) from the isomerization step(s) and/or various separatingsteps. The C₈+ hydrocarbon feedstock comprises PX, together with MX, OX,and/or EB. In addition to xylenes and EB, the C₈+ hydrocarbon feedstockmay also contain certain amounts of other aromatic or even non-aromaticcompounds. Examples of such aromatic compounds are benzene, toluene andC₉+ aromatics such as mesitylene, pseudo-cumene and others. These typesof feedstream(s) are described in “Handbook of Petroleum RefiningProcesses”, Eds. Robert A. Meyers, McGraw-Hill Book Company, SecondEdition, all relevant parts of which are hereby incorporated byreference.

Process Description

The process of the present application comprises an initial separatingstep that serves to remove the C₉+ hydrocarbons from the C₈+ hydrocarbonfeedstock. Because of the differences in molecular weights, boilingpoints and other physical and chemical properties, the C₉+ hydrocarbonscompounds, aromatic or non-aromatic, can be separated relatively easilyfrom the xylenes and EB. Generally, therefore, the first separating stepincludes fractional distillation, although other separation methods,such as crystallization, adsorption, a reactive separation, a membraneseparation, extraction, or any combination thereof, can also be used.These separation methods are described in “Perry's Chemical Engineers'Handbook”, Eds. R. H. Perry, D. W. Green and J. O. Maloney, McGraw-HillBook Company, Sixth Edition, 1984, and “Handbook of Petroleum RefiningProcesses”, Eds. Robert A. Meyers, McGraw-Hill Book Company, SecondEdition, all relevant parts of which are hereby incorporated byreference.

After removal of the C₉+ hydrocarbons, the present process comprises atleast one separating step to recover a PX-rich product stream from theresultant C₈ hydrocarbon stream. In one embodiment, the PX-rich productstream comprises at least 50 wt. % PX, preferably at least 60 wt. % PX,more preferably at least 70 wt. % PX, even preferably at least 80 wt. %PX, still even preferably at least 90 wt. % PX, and most preferably atleast 95 wt. % PX, based on the total weight of the PX-rich productstream. The separating step to recover the PX-rich product stream isperformed in a PX recovery unit comprising at least one acrystallization unit, an adsorption unit such as a PAREX™ unit or anELUXYL™ unit, a reactive separation unit, a membrane separation unit, anextraction unit, a distillation unit, a fractionation unit, or anycombination thereof. These types of separation unit(s) and their designsare described in “Perry's Chemical Engineers' Handbook”, Eds. R. H.Perry, D. W. Green and J. O. Maloney, McGraw-Hill Book Company, SixthEdition, 1984, and “Handbook of Petroleum Refining Processes”, Eds.Robert A. Meyers, McGraw-Hill Book Company, Second Edition, all relevantparts of which are hereby incorporated by reference.

Further separating steps employed in the present process serve toseparate a C₈ hydrocarbon feedstream into a PX-rich effluent stream anda PX-depleted stream. These separating steps are performed in separatingunits comprising at least one of a crystallization unit, an adsorptionunit such as a PAREX™ unit or an ELUXYL™ unit, a reactive separationunit, a membrane separation unit, an extraction unit, a distillationunit, a fractionation unit, or any combination thereof. These types ofseparation unit(s) and their designs are described in “Perry's ChemicalEngineers' Handbook”, Eds. R. H. Perry, D. W. Green and J. O. Maloney,McGraw-Hill Book Company, Sixth Edition, 1984, and “Handbook ofPetroleum Refining Processes”, Eds. Robert A. Meyers, McGraw-Hill BookCompany, Second Edition, all relevant parts of which are herebyincorporated by reference.

The process of the present application also comprises at least twoisomerization steps, in each of which a feed stream comprising C₈aromatic compounds is isomerized to produce an isomerization effluent.The feed stream to each isomerization step comprises PX in aconcentration below its equilibrium concentration relative to otherinter-convertible C₈ aromatic compounds under the isomerizationconditions. Each catalyzed isomerization step serves to increase the PXconcentration to near its equilibrium level. The isomerization step mayalso serve to convert part or all of EB present in the feed stream tobenzene and light hydrocarbons (i.e., hydrocarbons having less than 6carbons per molecule). Alternatively, the isomerization step may alsoserve to isomerize part or all of EB present in the feed stream toxylene(s).

There are many catalysts or combinations of catalysts that can be usedin each isomerization step to effect the desired reaction. There aregenerally two types of xylene isomerization catalysts. One type ofisomerization catalyst can more or less equilibrate the four differentC₈ aromatic compounds, including EB, to the concentrations dictated bythermodynamics under the reaction conditions. This allows maximumformation of PX from C₈ aromatics in a particular feed. Examples ofthese type catalysts include IFP/Engelhard Octafining™ and OctafiningII™ catalysts used in the respective processes. The other type of xyleneisomerization catalyst can effect EB conversion in addition to xyleneisomerization, generally in the presence of hydrogen. As discussedearlier, this type of catalyst will remove EB and produce benzene andethane as byproducts. This may be a desirable disposition of EB,depending on supplies and demands of various products as well as otherequipment present in a particular plant. Examples include Mobil HighTemperature Isomerization (MHTI™) catalysts, Mobil High ActivityIsomerization catalysts (MHAI™) and UOP ISOMAR™ I-100 catalysts.

A number of suitable isomerization reactors may be used for the presentdisclosure. Some non-limiting examples are described in U.S. Pat. Nos.4,899,011 and 4,236,996.

For the present disclosure, a xylene isomerization reaction may becarried out in a liquid phase, a vapor (gas) phase, a super criticalphase, or a combination thereof. The selection of isomerization reactionconditions and the specific composition of the aromatic feed streambeing isomerized determine the physical state of the aromatic feedstream in the xylene isomerization reactor.

Referring to FIG. 1, in the conventional para-xylene separation andisomerization loop shown, a feed comprising C₈+ aromatic hydrocarbons isdirected via line 1 to a separation unit 5, which is typically adistillation column. A majority of C₈ aromatic hydrocarbons in the feedis separated by the unit 5 and withdrawn via a line 10, while a majorityof C₉+ hydrocarbons in the feed is withdrawn via a line 40 as a bottomstream for further processing. The C₈ aromatic hydrocarbons streamwithdrawn via line 10 is supplied to a PX recovery unit 15 where aportion of PX in the stream is removed via line 20. The PX depletedeffluent from the PX recovery unit 15 is withdrawn via a line 25 and issupplied to an isomerization unit 30. The isomerization unit 30 isnormally a reactor or a vessel loaded with an isomerization catalyst(e.g., acidic zeolite) and operated under suitable isomerizationconditions sufficient to convert the PX depleted stream into anisomerized stream having a higher PX concentration than the PXconcentration of the PX depleted stream. The isomerized stream iswithdrawn from the isomerization unit 30 and recycled to the separationunit 5 via line 35.

Referring to FIG. 2 which describes one embodiment of the presentprocess, a feedstock comprising C₈+ aromatic hydrocarbons is directedvia line 101 to a first separation unit 105. The first separation unit105 may be any unit capable of separating C₈ aromatic hydrocarbons froma feedstock comprising C₈+ aromatic hydrocarbons and typically is adistillation column. A majority of C₈ aromatic hydrocarbons in thefeedstock is separated by the unit 105 and withdrawn via a line 110,while a majority of C₉+ hydrocarbons in the feed is withdrawn via a line140 as a bottom stream for further processing. At least a portion of theC₈ aromatic hydrocarbon stream withdrawn via line 110 is supplied to aPX recovery unit 115 where a portion of PX in the feed is withdrawn as aPX-rich stream via a line 120 and a first stream (PX depleted stream) iswithdrawn via a line 125.

At least a portion of the first stream withdrawn from the PX recoveryunit 115 via line 125 is supplied to a first isomerization unit 130. Thefirst isomerization unit 130 is normally a reactor loaded with anisomerization catalyst (e.g., acidic zeolite) and is operated undersuitable isomerization conditions sufficient to convert the first streaminto a second stream having a higher PX concentration than the PXconcentration of the first stream. The second stream is withdrawn fromthe first isomerization unit 130 and at least a portion thereof issupplied via line 136 to a second separation unit 137, where the secondstream supplied is separated into a third stream, having a higher PXconcentration than the second stream, and a fourth stream, having alower PX concentration than the second stream.

The fourth stream is withdrawn from the second separation unit 137 vialine 138 and at least a portion thereof is supplied to a secondisomerization unit 139. The second isomerization unit 139 is normally areactor or a vessel loaded with an isomerization catalyst (e.g., acidiczeolite) and operated under suitable isomerization conditions sufficientto convert the fourth stream into a fifth stream having a higher PXconcentration than the PX concentration of the fourth stream. The fifthstream is withdrawn from the second isomerization unit 139 via line 142and is combined with the third stream, which is withdrawn from thesecond separation unit 137 via line 141. At least a portion of the fifthstream and/or at least a portion of the third stream are jointly fed tothe first separation unit 105 via line 135.

In a modification (not shown) of the process of FIG. 2, a portion of thefifth stream in line 142 and/or a portion of the third stream in line141 may be recycled to the second separation unit 137. Further, theprocess may comprise recycling at least a portion of the fourth streamin line 138 to the first isomerization unit 130.

In a further modification (not shown) of the process of FIG. 2, thesecond stream withdrawn from the first isomerization unit 130 via line136 is fed to a fractionator before being supplied to the secondseparation unit 137. The fractionator divides the second stream into afirst portion rich in C₇− hydrocarbons, which is withdrawn for furtherprocessing, and a second portion rich in C₈+ hydrocarbons, which issupplied to the second separation unit 137.

Referring to FIG. 3, which describes another embodiment of the presentprocess, a feedstock comprising C₈+ aromatic hydrocarbons is directedvia line 201 to a first separation unit 205. The first separation unit205 may be any unit capable of separating C₈ aromatic hydrocarbons froma feedstock comprising C₈ and C₉+ aromatic hydrocarbons, again normallya distillation column. A majority of C₈ aromatic hydrocarbons in thefeedstock is separated by the unit 205 and withdrawn via a line 206,while a majority of C₉+ hydrocarbons in the feed is withdrawn via a line240 as a bottom stream for further processing. At least a portion of theC₈ aromatic hydrocarbon stream withdrawn via line 206 is supplied to asecond separation unit 207, which may be any unit capable of separatingor enriching PX from a feedstock comprising C₈ aromatic hydrocarbons.

The second separation unit 207 separates a portion of the C₈ aromatichydrocarbon stream withdrawn a line 206 into a first stream, having ahigher PX concentration than the C₈ aromatic hydrocarbon stream and asecond stream, having a lower PX concentration than the C₈ aromatichydrocarbon stream. The second stream is withdrawn from the firstseparation unit 207 via line 208 and at least a portion thereof issupplied to a first isomerization unit 209. The first isomerization unit209 is normally a reactor loaded with an isomerization catalyst (e.g.,acidic zeolite) and operated under suitable isomerization conditionssufficient to convert second stream into a third stream having higher PXconcentration than the PX concentration of the second stream. The thirdstream is withdrawn from the first isomerization unit 209 via line 212and is combined with the first stream, which is withdrawn from thesecond separation unit 207 via line 211.

At least a portion of the third stream and/or at least a portion of thefirst stream are jointly fed via line 210 to a PX recovery unit 215where a portion of PX in the joint stream is removed via line 220 as aPX-rich stream and a fourth stream (PX depleted stream) is withdrawn viaa line 225. At least a portion of the fourth stream is supplied via line225 to a second isomerization unit 230. The second isomerization unit230 is normally a reactor loaded with an isomerization catalyst (e.g.,acidic zeolite) and operated under suitable isomerization conditionssufficient to convert the fourth stream into a fifth stream havinghigher PX concentration than the PX concentration of the fourth stream.The fifth stream is withdrawn from the second isomerization unit 230 andat least a portion thereof is supplied to the first separation unit 205via line 235.

In a modification (not shown) of the process of FIG. 3, a portion of thesecond stream in line 208 may be supplied to the second separation unit230. In addition, a portion of the first stream in line 211 and/or aportion of the third stream in line 212 may be recycled to the secondseparation unit 207.

In a further modification (not shown) of the process of FIG. 3, at leasta portion of the third stream in line 212 may be supplied to the firstseparation unit 205.

In yet a further modification (not shown) of the process of FIG. 3, thefifth stream withdrawn from the second isomerization unit 230 via line235 is fed to a fractionator before being supplied to the firstseparation unit 207. The fractionator divides the fifth stream into afirst portion rich in C₇— hydrocarbons, which is withdrawn for furtherprocessing, and a second portion rich in C₈+ hydrocarbons, which issupplied to the second separation unit 207.

Referring now to FIG. 4, a further known xylene production process isshown that integrates selective adsorption and a fractionalcrystallization unit in a single para-xylene separation andisomerization loop. In particular, the process comprises a comprises afirst separating unit 301, which may be one or more distillation columnsand which receives a C₈+ aromatic hydrocarbon feed stream from line 302and separates the feed into an overhead vapor stream and a bottom liquidstream. The bottom liquid stream is composed mainly of C₉+ aromatichydrocarbons and some ortho-xylene (OX) and is removed from the firstseparating unit 301 through line 303 for further processing. Theoverhead stream is composed mainly of C₈ aromatic hydrocarbons(typically about 50% meta xylene (MX), about 20% PX, about 15% OX andabout 15% EB) and is removed from the first separating unit 301 throughline 304 and is sent for PX recovery.

PX recovery in the process shown in FIG. 4 is effected by both afractional crystallization unit 308 and a selective adsorption unit 309.Thus part of the C₈ aromatic hydrocarbon removed from the firstseparating unit 301 through line 304 is fed by line 306 to thefractional crystallization unit 308, where a first PX-rich productstream is recovered through line 310 and a PX-depleted raffinate streamis withdrawn via line 311. The remainder of the C₈ aromatic hydrocarbonremoved from the first separating unit 301 through line 304 is combinedwith the PX-depleted raffinate stream from the fractionalcrystallization unit 308 and fed by line 312 to the selective adsorptionunit 309, where a second PX-rich product stream is recovered throughline 313 and a further PX-depleted stream is withdrawn via line 314. Thefurther PX-depleted stream is fed by line 314 to a xylene isomerizationunit 315 where the further stream is converted into an isomerized streamhaving higher PX concentration than that of the further stream. Theisomerized stream is removed from the xylene isomerization unit 315 byline 316 and is fed to line 302 for recycle to the splitter 301.

Referring to FIG. 5, a modification of the known technology of FIG. 4 isshown in which the present process is used to increase the PXproductivity of the para-xylene separation and isomerization loop. Likereference numerals are therefore used to illustrate like components inFIGS. 4 and 5. In particular the process of Example 5 employs a secondxylene isomerization unit 317 which is used to treat the PX-depletedraffinate stream withdrawn from the fractional crystallization unit 308via line 311. Thus a problem with the known process shown in FIG. 4 isthe low PX concentration in the crystallizer raffinate stream, whichreduces the productivity of the selective adsorption unit 309 since, ineach pass, the unit 309 can only recover the PX being fed through line312. The purpose of the treatment in the second xylene isomerizationunit 316 is to bring the PX concentration in the PX-depleted raffinatestream from 10-12% up to the equilibrium level of 20-24%.

In the process shown in FIG. 5, the effluent stream from the secondxylene isomerization unit 317, which has a higher PX concentration thanthe PX-depleted raffinate stream, is withdrawn through line 318 and fedto the first separating unit 301. In this way, the overall PX content ofthe feed to the selective adsorption unit 309 can be increased. It is,however, to be appreciated that part or all of the effluent stream fromsecond xylene isomerization unit 317 could be supplied directly to theselective adsorption unit 309, if necessary after fractionation toremove C₉+ hydrocarbon impurities and/or C₇− hydrocarbon impuritiesgenerated in the second xylene isomerization unit 317.

In one practical embodiment of the process shown in FIG. 5, the secondxylene isomerization unit 316 employs liquid phase isomerizationtechnology since this has the advantages of (1) simplicity and cost,since, unlike the more common, gas phase, high temperature isomerizationtechnologies, liquid phase isomerization does not require hydrogenrecycle; and (2) low xylene loss (<1.0%) due to the low levels ofundesirable side reactions at the more moderate reaction conditions.

Because the liquid isomerization product contains mostly equilibriumxylenes and low levels of C₉+ compounds, it is possible to send theeffluent stream from the second xylene isomerization unit 317 directlyto the first separating unit 301 at the proper tray position to affectthe separation of C₈ and C₉+ compounds.

EXAMPLES

The following simulation examples were performed based on the followingassumptions:

-   -   (a) the isomerization unit(s) isomerizes PX, MX, and OX to their        thermodynamic equilibrium;    -   (b) the equilibrium PX concentration in xylenes (excluding EB)        is 25%;    -   (c) the isomerization unit(s) converts all EB to benzene,        toluene, xylenes, or other hydrocarbons;    -   (d) the PX recovery unit recovers 100% of PX in its feed; and    -   (e) the first separation unit is a distillation column which        recovers all xylenes in the combined feed.

It is understood to a person skilled in the art that the isomerizationunit(s) may isomerize PX, MX, and OX to less than 100% thermodynamicequilibrium concentration in real manufacturing plants. It is alsounderstood to a person skilled in the art that the PX equilibriumconcentration in xylenes (excluding EB) is usually less than 25%. It isagain understood to a person skilled in the art that the isomerizationunit(s) may convert less than 100% EB to other hydrocarbons. It isfurther understood to a person skilled in the art that the PX recoveryunit may recover less than 100% of PX in its feed in real manufacturingplants. However, for the purpose of simplicity, 100% xylene equilibrium,100% EB conversion, 100% PX recovery, and 25% PX equilibriumconcentration are assumed in the following Examples.

Comparative Example 1

As shown in the simplified schematic diagram of a conventional processfor producing PX in FIG. 1, one unit of xylenes in a C₈+ aromaticfeedstock (via line 1) is combined with a recycle stream (via line 35)having 3 units of xylenes and is directed to the separation unit 5(distillation column). The majority of C₉+ hydrocarbons are separatedfrom the combined feed by the distillation column 5 and withdrawn vialine 40 as a bottom stream for further processing. The overhead streamfrom the distillation column 5 contains the majority of C₈ aromatichydrocarbons (xylenes and EB) in the feed and has four units of totalxylenes, of which 25% (1 unit) is PX. The overhead stream is withdrawnvia line 10 and supplied to the PX recovery unit 15 via line 10. The PXrecovery unit used in this example is a PAREX™ unit. One unit of the PXin the overhead stream is removed via line 20 as a PX-rich stream havinga PX concentration of about 99.6 to 99.9 wt. % PX based on the totalweight of the PX-rich stream. A PX depleted stream is withdrawn recoveryunit 15 via line 25 and fed to the isomerization unit 30. In thisexample, the isomerization unit isomerizes MX and OX to PX; and convertsEB to mainly benzene and other hydrocarbons. The isomerized streamhaving about 3 units of xylenes and a PX concentration (among totalxylenes) of about 25 wt. % is withdrawn from the isomerization unit 30and recycled back to the separation unit 5 via line 35.

Example 1

As shown in the simplified schematic diagram of one embodiment of thisdisclosure (FIG. 2), one unit of xylenes in a C₈+ aromatic feedstock iscombined with a recycle stream having 2 units of xylenes and is directedvia line 1 to the first separation unit 105 (distillation column). Themajority of C₉+ hydrocarbons are separated from the combined feed by thedistillation column 105 and withdrawn via line 140 as a bottom streamfor further processing. The overhead stream of the distillation column105 contains the majority of C₈ aromatic hydrocarbons (xylenes and EB)in the feed and has 3 units of total xylenes, of which about 33.3% arePX. The overhead stream is withdrawn via line 110 and supplied to the PXrecovery unit 115, which in this example is a PAREX™ unit. One unit ofthe PX in the overhead stream is removed via line 120 as a PX-richstream having a PX concentration of about 99.6 to 99.9 wt. % based onthe total weight of the PX-rich stream. A PX depleted stream iswithdrawn from unit 115 via line 125 and fed to the first isomerizationunit 130. In this example, the first isomerization unit isomerizes MXand OX to PX; and converts EB to mainly benzene and other hydrocarbons.The isomerized stream from the first isomerization unit 130 having about2 unit of xylenes and a PX concentration (among total xylenes) of about25 wt. % is withdrawn from the isomerization unit 130.

The isomerized stream from the first isomerization unit 130 is suppliedvia line 136 to the second separation unit 137 and separated into astream having a higher PX concentration than the isomerized stream fromthe first isomerization unit 130 and a stream having a lower PXconcentration than the isomerized stream from the first isomerizationunit 130. The stream having a lower PX concentration than the isomerizedstream from the first isomerization unit 130 is withdrawn from thesecond separation unit 137 via line 138 and supplied to the secondisomerization unit 139. The product stream from the second isomerizationunit 139 is withdrawn via line 142 and combined with the stream having ahigher PX concentration than the isomerized stream from the firstisomerization unit 130 to form a combined product withdrawn via line135. The second isomerization unit 139 and the second separation unit137 are operated so that the combined product (in line 135) has a PXconcentration of about 37% among total xylenes. The combined product isrecycled back to the separation unit 105 via line 135.

It will be seen that the ratio of the recycle stream (combined productvia line 135) to the feed stream (via line 101) in Example 1 is 2:1,down from the 3:1 recycle ratio in Comparative Example 1, whichdebottlenecks the existing xylene loop.

Example 2

As show in the simplified schematic diagram of one embodiment of thisdisclosure (FIG. 3), one unit of xylenes in a C₈+ aromatic feedstock fedvia line 201 is combined with a recycle stream having 2 units of xylenesfed via line 235 and is directed to the first separation unit 205(distillation column). The majority of C₉+ hydrocarbons in the feed areseparated in the distillation column 205 and withdrawn via line 240 as abottom stream for further processing. The overhead stream from thedistillation column contains the majority of C₈ aromatic hydrocarbons(xylenes and EB) in the feed and has 3 units of total xylenes. Theoverhead stream is withdrawn from the distillation column via line 206and supplied to the second separation unit 207 here the overhead streamis separated into a stream having a higher PX concentration than theoverhead stream in line 211 and a stream having a lower PX concentrationthan the overhead stream in line 208. The lower PX concentration streamin line 208 is supplied to the first isomerization unit 209 and isisomerized to produce an effluent stream which is combined with thehigher PX concentration stream in line 211. The first isomerization unit209 and the second separation unit 207 are operated so that the combinedproduct (in line 210) has a PX concentration of about 33.3% based ontotal xylenes.

The combined product in line 210 is supplied to the PX recovery unit215, which in this example is a PAREX™ unit. One unit of the PX in theoverhead stream is removed via line 220 as a PX-rich stream, which has aPX concentration of about 99.6 to 99.9 wt. % PX based on the totalweight of the PX-rich stream. A PX depleted stream is withdrawn from thePX recovery unit 215 via a line 225 and enters the second isomerizationunit 230. In this example, the second isomerization unit isomerizes MXand OX to PX; and converts EB to mainly benzene and other hydrocarbons.The isomerized stream from the second isomerization unit 230 has about 2unit of xylenes and a PX concentration (among total xylenes) of about 25wt. % and is recycled back to the separation unit 205 via line 135.

Again it will be seen that the ratio of the recycle stream (via line235) to the feed stream (via line 201) in Example 1 is 2:1, down fromthe 3:1 recycle ratio in Comparative Example 1, which debottlenecks theexisting xylene loop.

The arrangements in Examples 1 and 2 reduce the recycle/feed ratio to 2.Assuming an original capacity of 4 units of xylenes in the xylene loopand assuming this capacity is fully utilized, a 1.33 unit of feed and a2.66 unit recycle satisfy the recycle-to-feed ratio of 2 and completelyfill up the original capacity of 4 units of feed to the PX recoveryunit. A total 1.33 units of PX is recovered from the PX recovery unitcompared to the one (1) unit of feed in the conventional PX plant asshown in the Comparative Example, thereby demonstrating a 33% increasein PX production capacity (1.33 from 1.0).

Comparative Example 2

In the simulation of the known process shown in FIG. 4, it is assumedthat 1.2 units of xylenes in a C₈+ aromatic feedstock fed via line 302are combined with a recycle stream fed having 4.3 units of xylenes fedvia line 316 to give a total xylenes feed to the first separating unit301 of 5.5 units of which 2.4 units of the overhead stream are sent tothe selective adsorption unit 309 and 3.1 units are sent to thefractional crystallization unit 308 via line 306. The fractionalcrystallization unit 308 separates 0.4 units of high purity PX (>99.5%)from the condensed liquid in the line 306 leaving 2.7 units ofPX-depleted raffinate, which contains 10-12% PX and mostly other xyleneisomers.

The 2.7 units of the PX-depleted raffinate from fractionalcrystallization unit 308 are combined with the remaining 2.4 units ofcondensed liquid from the condenser 305 to give a total feed of 5.1units to the selective adsorption unit 309. Because the PX-depletedraffinate has only 10-12% PX, the mixture, which is sent to selectiveadsorption unit 309, typically has a PX concentration of about 15-18%.The selective adsorption unit 309 in this Example is a PAREX unit andproduces 0.8 units of high purity PX (>99.5%) and 4.3 units of a furtherPX-depleted raffinate stream. The further PX-depleted raffinate streamis supplied to the xylene isomerization unit 315 which generates 4.3units of xylenes at 24-25% PX concentration, which is recycled back tothe splitter 301.

Example 3

In the simulation of the process shown in FIG. 5, the total xylenes feedto the first separating unit 301 remains at 5.5 units but is composed of1.4 units of xylenes in the C₈+ aromatic feedstock fed via line 302 and4.1 units of xylenes in the recycle stream fed via line 316. Theoverhead stream from the first separating unit 301 again comprises 5.5units of xylenes. As in the case of Comparative Example 2, 3.1 units ofthe overhead stream are sent to the fractional crystallization unit 308,where 0.4 units of high purity PX (>99.5%) are recovered leaving 2.7units of PX-depleted raffinate. However, in Example 3, the PX-depletedraffinate is fed to the liquid phase isomerization reactor 317, whichincreases the PX concentration of the raffinate to about 23 wt %. Theresultant 2.7 units of isomerized raffinate are recycled to the firstseparating unit 301. The overhead stream in line 304 therefore has atotal of 8.2 units (5.5 units+2.7 units), of which 5.1 units aresupplied via line 312 to the PAREX unit 309, but in this case the PXconcentration of the feed to the PAREX unit is 20-24 wt % (as comparedwith the 15-18 wt % of Comparative Example 2), which effectively allowsa 25% PAREX productivity increase from 0.8 units to 1.0 units of PX. Asa result, the PAREX raffinate stream in line 314 is reduced from 4.3units to 4.1 units saving energy as the amount of recycle is reduced.The fresh feed has increased from 1.2 units to 1.4 units to meet the newcapacity. However, the total quantity of molecules going into the firstseparating unit 301 (5.5 units), the crystallizer (3.1 units), and thePAREX unit (5.1 units) all remain unchanged. Thus, the proposedtechnology debottlenecks the whole loop with a minimum amount of newequipment.

While the present invention has been described and illustrated byreference to particular embodiments, those of ordinary skill in the artwill appreciate that the invention lends itself to variations notnecessarily illustrated herein. For this reason, then, reference shouldbe made solely to the appended claims for purposes of determining thetrue scope of the present invention.

1. A process for producing a PX-rich product, the process comprising:(a) separating a feedstock containing C₈ hydrocarbons to produce a C₈hydrocarbons rich stream; (b) separating at least a first portion of theC₈ hydrocarbons rich stream to produce a first PX-rich stream and afirst PX-depleted stream; (c) isomerizing at least a portion of thefirst PX-depleted stream to produce a first isomerized stream having ahigher PX concentration than the first PX-depleted stream; (d)separating a second portion of the C₈ hydrocarbons rich stream and atleast a portion of the first isomerized stream to produce a secondPX-rich stream and a second PX-depleted stream; (e) isomerizing at leasta portion of the second PX-depleted stream to produce a secondisomerized stream having a higher PX concentration than the secondPX-depleted stream; (f) recovering at least a portion of the first andsecond PX-rich streams as PX-rich product; and (g) supplying at least aportion of the second isomerized stream to the separating (a).
 2. Theprocess recited in claim 1, wherein the feedstock contains at least C₈+hydrocarbons and the separating (a) produces the C₈ hydrocarbons richstream and a C₉+ hydrocarbons rich stream.
 3. The process recited inclaim 1, wherein each PX-rich stream comprises at least 50 wt. % PX. 4.The process recited in claim 1, wherein each PX-rich stream comprises atleast 90 wt. % PX.
 5. The process recited in claim 1, wherein saidseparating (a) comprises distillation of said feedstock.
 6. The processrecited in claim 1, wherein said separating (b) comprises at least oneof selective adsorption, selective crystallization, selectiveextraction, and selective membrane separation.
 7. The process recited inclaim 1, wherein said separating (d) comprises at least one of selectiveadsorption, selective crystallization, selective extraction, andselective membrane separation.
 8. The process recited in claim 1,wherein said isomerizing (c) is carried out in an isomerization unitcontaining a catalyst operated under isomerization conditions sufficientto isomerize MX and/or OX to PX and/or to convert EB to benzene and/orxylenes.
 9. The process recited in claim 1, wherein said isomerizing (e)is carried out in an isomerization unit containing a catalyst operatedunder isomerization conditions sufficient to isomerize MX and/or OX toPX and/or to convert EB to benzene and/or xylenes.
 10. A process forproducing a PX-rich stream, comprising the steps of: (a) separating afeedstock containing C₈ hydrocarbons to produce a C₈ hydrocarbons richstream; (b) separating at least a portion of said C₈ hydrocarbons richstream to produce said PX-rich stream and a first stream; (c)isomerizing at least a portion of said first stream to produce a secondstream having a higher PX concentration than said first stream; (d)separating at least a portion of said second stream to produce a thirdstream and a fourth stream, said third stream having a higher PXconcentration than said second stream and said fourth stream having alower PX concentration than said second stream; (e) isomerizing at leasta portion of said fourth stream in another isomerizing step separatedfrom said isomerization step (c) to produce a fifth stream having ahigher PX concentration than said fourth stream; and (f) providing atleast a portion of said third stream and at least a portion of saidfifth stream to said separating step (a).
 11. The process recited inclaim 10, and further comprising recycling at least a portion of saidfifth stream and/or at least a portion of said third stream to step (d).12. The process recited in claim 10, and further comprising recycling atleast a portion of said fourth stream to step (c).
 13. The processrecited in claim 10, and further comprising fractionating said secondstream to produce a first portion rich in C₇− hydrocarbons and a secondportion rich in C₈+ hydrocarbons, said second portion being supplied tosaid separating (d).
 14. The process recited in claim 10, wherein saidPX-rich stream comprises at least 50 wt. % PX.
 15. The process recitedin claim 10, wherein said PX-rich stream comprises at least 90 wt. % PX.16. The process recited in claim 10, wherein said separating (a)comprises distillation of said feedstock.
 17. The process recited inclaim 10, wherein said separating (b) comprises at least one ofselective adsorption, selective crystallization, selective extraction,and selective membrane separation.
 18. The process recited in claim 10,wherein said separating (d) comprises at least one of selectiveadsorption, selective crystallization, selective extraction, andselective membrane separation.
 19. The process recited in claim 10,wherein said isomerizing (c) is carried out in an isomerization unitcontaining a catalyst operated under isomerization conditions sufficientto isomerize MX and/or OX to PX and/or to convert EB to benzene and/orxylenes.
 20. The process recited in claim 10, wherein said isomerizing(e) is carried out in an isomerization unit containing a catalystoperated under isomerization conditions sufficient to isomerize MXand/or OX to PX and/or to convert EB to benzene and/or xylenes.
 21. Aprocess for producing a PX-rich stream, comprising the steps of: (a)separating a feedstock containing C8 hydrocarbons to produce a C₈hydrocarbons rich stream; (b) separating at least a portion of said C₈hydrocarbons rich stream to produce a first stream and a second stream,said first stream having a higher PX concentration than said C₈hydrocarbons rich stream and said second stream having a lower PXconcentration than said C₈ hydrocarbons rich stream; (c) isomerizing atleast a portion of said second stream to produce a third stream having ahigher PX concentration than said second stream; (d) separating at leasta portion of said first stream and at least a portion of said thirdstream to produce said PX-rich stream and a fourth stream; (e)isomerizing at least a portion of said fourth stream to produce a fifthstream having a higher PX concentration than said fourth stream; and (f)providing at least a portion of said fifth stream to said separatingstep (a).
 22. The process recited in claim 21, and further comprisingrecycling at least a portion of said second stream to step (e).
 23. Theprocess recited in claim 21, and further comprising recycling at least aportion of said first stream and/or at least a portion of said thirdstream to step (b).
 24. The process recited in claim 21, and furthercomprising recycling at least a portion of the third stream to step (a).25. The process recited in claim 21, and further comprisingfractionating said fifth stream to produce a first portion rich in C₇−hydrocarbons and a second portion rich in C₈+ hydrocarbons, said secondportion being supplied to said separating (a).
 26. The process recitedin claim 21, wherein said PX-rich stream comprises at least 50 wt. % PX.27. The process recited in claim 21, wherein said PX-rich streamcomprises at least 90 wt. % PX.
 28. The process recited in claim 21,wherein said separating (a) comprises distillation of said feedstock.29. The process recited in claim 21, wherein said separating (b)comprises at least one of selective adsorption, selectivecrystallization, selective extraction, and selective membraneseparation.
 30. The process recited in claim 21, wherein said separating(d) comprises at least one of selective adsorption, selectivecrystallization, selective extraction, and selective membraneseparation.
 31. The process recited in claim 21, wherein saidisomerizing (c) is carried out in an isomerization unit containing acatalyst operated under isomerization conditions sufficient to isomerizeMX and/or OX to PX and to convert EB to benzene and/or xylenes.
 32. Theprocess recited in claim 21, wherein said isomerizing (e) is carried outin an isomerization unit containing a catalyst operated underisomerization conditions sufficient to isomerize MX and/or OX to PX andto convert EB to benzene and/or xylenes.
 33. A process for producing aPX-rich product, the process comprising: (a) separating a feedstockcontaining C₈ hydrocarbons to produce a C₈ hydrocarbons rich stream; (b)separating a first portion of the C₈ hydrocarbons rich stream to producea first PX- rich stream and a first stream; (c) isomerizing at least aportion of the first stream to produce a second stream having a higherPX concentration than the first stream; (d) separating a second portionof the C₈ hydrocarbons rich stream to produce a second PX-rich streamand a third stream; (e) isomerizing at least a portion of the thirdstream to produce a fourth stream having a higher PX concentration thanthe third stream; (f) recovering at least a portion of the first andsecond PX-rich streams as PX-rich product; and (g) providing at least aportion of the second stream and at least a portion of the fourth streamto the separating (a).
 34. The process recited in claim 33, wherein saidisomerizing (c) is carried out in an isomerization unit containing acatalyst operated under isomerization conditions sufficient to isomerizeMX and/or OX to PX and/or to convert EB to benzene and/or xylenes. 35.The process recited in claim 32, wherein said isomerizing (e) is carriedout in an isomerization unit containing a catalyst operated underisomerization conditions sufficient to isomerize MX and/or OX to PXand/or to convert EB to benzene and/or xylenes.
 36. The process recitedin claim 33, wherein said isomerizing (e) is effected at least partiallyin the liquid phase.
 37. The process recited in claim 33, wherein saidseparating (b) comprises selective adsorption.
 38. The process recitedin claim 33, wherein said separating (d) comprises fractionalcrystallization.